The present invention relates to a copier, facsimile, printer or similar image forming apparatus and, more particularly, to a developing device for use in an image forming apparatus and including a developing roller or similar developer carrier having dielectric portions and conductive portions, which are connected to ground, distributed regularly or irregularly over the surface thereof.
In an image forming apparatus, a latent image is electrostatically formed carrier and then on an image developed by a developer. The developer is advantageously implemented by a single component type developer, or toner, since this type of developer miniaturizes a developing apparatus, reduces cost, and enhances reliability. Particularly, for color development, a nonmagnetic single component type developer, which has inherently high transparency, is desirable. The developing device using this kind of developer may include a developer carrier for carrying the developer on the surface thereof and conveying it along a predetermined circulation path, including a developing region, developer storing means for storing the developer, and a developer supply member for supplying the developer from the storing means to the developer carrier, as taught in, for example, Japanese Laid-Open Patent Publication Nos. 60-229057 and 61-42672.
In the developing device using a nonmagnetic single component type developer, or toner as referred to hereinafter, the toner and a charge should each be deposited in an optimal amount on the developer carrier, as follows. The toner should preferably be deposited on the developer carrier such that it is transferred to the image carrier in an amount of about 0.6 mg/cm.sup.2 to 1.0 mg/cm.sup.2 and to a paper or similar recording medium in an amount of 0.5 mg/cm.sup.2 to 0.7 mg/cm.sup.2. The amounts of toner as measured on the image carrier and paper depend not only on the amount of toner deposited on the developer carrier, but also on a difference in peripheral speed between the image carrier and the developer carrier in a developing region.
However, a problem with this type of conventional developing device is that the toner is deposited on the developer carrier only in a single layer, i.e., in an amount of 0.2 mg/cm.sup.2 to 0.5 mg/cm.sup.2. Hence, the desirable amounts of toner deposition on the image carrier and paper mentioned above cannot be attained unless the developer carrier is moved at a peripheral speed which is two to four times as high as that of the image carrier. However, increasing the rotation speed of the developer carrier makes it difficult to increase the image forming speed. Moreover, such a scheme brings about an undesirable occurrence that when a solid image having a substantial area is developed, the density increases at the rear edge portion of the image. Let this occurrence be referred to as toner offset hereinafter. The toner offset is not critical in a black-and-white image. However, when it comes to a color image whose colors are recognizable through toners, the density appears high in the rear edge portion and, in the case of a composite color image, results in a color different from the original color. In addition, the density becomes irregular around outline characters and prevents the entire image from appearing smooth.
To deposit a desired amount of toner on each of the image carrier and paper without causing the toner offset to occur, it is necessary that the peripheral speed of the developer carrier be brought close to that of the image carrier, i.e., nearly equispeed development be effected, and that a greater amount of toner be deposited on the developer carrier. Specifically, to insure the sufficient amounts of toner on the image carrier and paper by substantially equispeed development, the toner has to deposit on the developer carrier in an amount of at least 0.8 mg/cm.sup.2 in the case of contact development, which has a high developing efficiency, or in an amount of at least 1.0 mg/cm.sup.2 in the case of non-contact development whose developing efficiency is low. Such an amount of toner deposition on the developer carrier is not practicable unless it is deposited in two or more layers. Further, if the toner layer being conveyed toward the developing region by the developer carrier contains uncharged particles and reversely charged particles, it will degrade the transfer thereof to the image carrier, contaminate the background of an image, and lower resolution. In light of this, a charge should preferably be deposited on the toner in a mean amount of 5 .mu.C/g to 10 .mu.C/g. At the same time, since particles not sufficiently charged would lower sharpness and resolution and contaminate the background, the charge distribution on the toner should preferably be stable and contain hardly any particles of low charge.
As stated above, only if two or more toner layers are formed on the developer carrier with a mean amount of charge of 5 .mu.C/g to 10 .mu.C/g and in a stable charge distribution free from uncharged toners and reversely charged toners, it is possible to increase the image forming speed and effect equispeed development capable of eliminating the previously stated toner offset.
To implement stable multiple toner layers and convey them to the image carrier carrier, Japanese Patent Laid-Open Publication No. 2-15110, for example, proposes a developing device which forms fine closed electric fields, or so-called microfields, on the surface of the developer carrier. However, the problem with type of developing device is that when the amount of charge frictionally deposited on the toner decreases or the amount of charge induced decreases due to ambient temperature and/or humidity, the amount in which the toner is deposited on and conveyed by the developer carrier is apt to change, lowering image density.